The present invention relates to Coriolis-type mass flowmeters having one or more flowtubes through which a fluid to be measured is permitted to flow. More particularly, the invention relates to an apparatus and method for balancing the flowtube over all conditions and frequencies to which the flowtube may be subjected during operation of the flowmeter.
Prior art Coriolis mass flowmeters typically include one or more flowtubes through which a fluid to be measured is directed, a number of force drivers for vibrating the flowtube in one of its modes of vibration, and one or more sensors for measuring the vibratory deflections of the flowtube. During normal operation of the flowmeter, the flowtube experiences two types of vibratory deflections: a primary or driven deflection, which is induced by the force drivers through the application of sinusoidal forces at the natural frequency of the flowtube in, for example, its 1st bending mode of vibration, and a secondary or Coriolis deflection, which is induced by the Coriolis forces that are generated by the mass of the fluid flowing through the vibrating flowtube. When the flowtube is driven in its 1st bending mode of vibration, the resulting Coriolis deflection typically resembles the 2nd bending mode of vibration of the flowtube. In order to obtain the mass flow rate of the fluid, the Coriolis deflection is measured by the sensors, and from this measurement the flow rate of the fluid can be derived in a manner that is well understood by those skilled in the art.
The flowtube of the typical Coriolis mass flowmeter is securely connected to a supporting structure, such as the process piping, through end connections which are welded or otherwise attached to the flowtube. The other components of the flowmeter, such as the processing and control circuitry and the flowmeter housing, are likewise coupled to the flowtube through these end connections. The supporting structure and the other components of the flowmeter define the boundary conditions for the flowtube which typically influence the operation of the flowmeter. In the absence of an effective counterbalance, the vibrating flowtube will react against the supporting structure and the other components of the flowmeter through the end connections. The reaction forces created by the vibrating flowtube at the end connections can excite the supporting structure and the other components of the flowmeter and thereby drain energy from the vibrating flowtube and create erroneous flow rate readings. Therefore, the prior art has long recognized a need for a counterbalance mechanism to balance the flowtube during operation of the flowmeter.
The condition of balance between a flowtube and a counterbalance mechanism requires that the reaction forces from each be equal and opposite, thereby canceling each other at the point where the two structures are coupled together. This will result in the vibration of the system, that is, the vibration of the flowtube and the counterbalance, being contained and therefore isolated from any changes in the boundary conditions or the parameters of the fluid being measured. To achieve this condition of balance, the counterbalance must be able to change its natural frequency and frequency response to match those of the flowtube, which can change drastically in response to changing fluid parameters such as temperature, density, pressure and viscosity. Moreover, to achieve the best condition of balance the counterbalance should match the reaction forces at the ends of the flowtube by equal and opposite forces for not only the primary vibratory deflection of the flowtube, but also the secondary vibratory deflection resulting from the Coriolis forces.
The prior art has developed several counterbalance mechanisms in an attempt to effectively balance the flowtube. For example, one prior art flowmeter includes a fixed counterbalance which is similar in shape to the flowtube, is coupled to the flowtube proximate the end connections and includes weights to simulate the density of the fluid to be measured. Thus, when the flowtube is vibrated against this counterbalance, the reaction forces generated by the counterbalance will nullify the reaction forces from the flowtube at the end connections. However, this counterbalance is limited in its ability to match the flowtube over changes in the boundary conditions and the fluid parameters.
Another prior art flowmeter comprises two identical parallel flowtubes which are coupled at the end connections. The fluid to be measured is directed through both flow tubes and the flowtubes are vibrated in opposition to each other. Consequently, the tubes remain in near perfect balance regardless of changes in the parameters of the fluid being measured. However, splitting the flow stream into two paths can create a pressure loss and turbulence in the flow stream and can also result in one flowtube becoming plugged.
The present invention addresses these and other limitations in the prior art by providing a counterbalance apparatus for a Coriolis-type mass flowmeter which includes a flowtube having first and second ends, the counterbalance apparatus comprising a counterbalance beam which is coupled to the flow tube proximate the first and second ends, means for vibrating the flowtube and the counterbalance beam in opposition to one another, at least one pair of inertial masses, each of which is movably coupled to the counterbalance beam, and means for selectively positioning the inertial masses along the length of the counterbalance beam. In this manner, the frequency response of the counterbalance beam can be selectively altered by appropriately positioning the inertial masses along the counterbalance beam to thereby achieve a desired condition of balance between the counterbalance beam and the flowtube.
In the preferred embodiment of the invention, the counterbalance beam comprises an elongated tube which, with the inertial masses coupled to the counterbalance beam in an initial position, is designed to have a mass and stiffness distribution along its length which are similar to those of the flowtube for a given condition, such as when the flowtube is filled with air. The counterbalance beam will therefore vibrate in a similar fashion, at the same frequency, and in opposition to the flowtube and thereby generate equal and opposite reaction forces at the first and second ends of the flowtube, which will result in a balanced condition between the counterbalance beam and the flowtube. If the flowmeter becomes unbalanced, for example in response to a change in the fluid parameters, the inertial masses can be moved to new positions along the counterbalance beam to thereby alter its natural frequency and frequency response as necessary to match that of the flowtube. By controlling the positions of the masses, the requisite condition of balance can thus be restored xe2x80x9con the flyxe2x80x9d over any desired range of fluid conditions.
In one embodiment of the invention, the counterbalance beam comprises a non-magnetic metal tube and each inertial mass comprises a cylindrical magnet which is slidably disposed within the tube. In addition, the inertial mass positioning means preferably comprises a number of electrical coils disposed around the counterbalance beam for generating electromagnetic fields which can selectively move the masses in order to position them for optimal balance. Furthermore, the counterbalance apparatus also preferably employs a feedback control means so that, as the natural frequency and frequency response of the flowtube change, the electrical coils can be activated to move the masses to new positions in order to maintain the counterbalance beam in optimal balance with the flowtube.
These and other objects and advantages of the present invention will be made apparent from the following detailed description, with reference to the accompanying drawings. In the drawings, the same reference numbers are used to denote similar components in the various embodiments.